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I put the black border on the main-container to test it fits inside the white border created by the mask. It doesn't. How can I get it to do this? I can't work out how I can create a % as the mask auto stretches to the screen size.

I withdraw my solution in #3 above, as it does not work. I've found a solution, but will just give a clue: It's to do with the HTML elements: HTML and BODY and the properties you can set for them in CSS. Many people do not realise that the HTML tag can have CSS properties set.